exp/template/html: fix JS regexp escape of an empty string.

R=dsymonds
CC=golang-dev, mikesamuel
https://golang.org/cl/4972063

src/pkg/exp/template/html/escape_test.go

@@ -194,8 +194,13 @@ func TestEscape(t *testing.T) {
 		},
 		{
 			"jsRe",
-			"<button onclick='alert(&quot;{{.H}}&quot;)'>",
-			`<button onclick='alert(&quot;\x3cHello\x3e&quot;)'>`,
+			`<button onclick='alert(/{{"foo+bar"}}/.test(""))'>`,
+			`<button onclick='alert(/foo\x2bbar/.test(""))'>`,
+		},
+		{
+			"jsReBlank",
+			`<script>alert(/{{""}}/.test(""));</script>`,
+			`<script>alert(/(?:)/.test(""));</script>`,
 		},
 		{
 			"styleBidiKeywordPassed",

src/pkg/exp/template/html/js.go

@@ -174,7 +174,12 @@ func jsStrEscaper(args ...interface{}) string {
 // expression literal. /foo{{.X}}bar/ matches the string "foo" followed by
 // the literal text of {{.X}} followed by the string "bar".
 func jsRegexpEscaper(args ...interface{}) string {
-	return replace(stringify(args...), jsRegexpReplacementTable)
+	s := replace(stringify(args...), jsRegexpReplacementTable)
+	if s == "" {
+		// /{{.X}}/ should not produce a line comment when .X == "".
+		return "(?:)"
+	}
+	return s
 }
 
 // stringify is an optimized form of fmt.Sprint.

src/pkg/exp/template/html/js_test.go

@@ -224,7 +224,7 @@ func TestJSRegexpEscaper(t *testing.T) {
 		x   interface{}
 		esc string
 	}{
-		{"", ``},
+		{"", `(?:)`},
 		{"foo", `foo`},
 		{"\u0000", `\0`},
 		{"\t", `\t`},